Method to provide a void between adjacent conducting lines in a semiconductor device

ABSTRACT

The invention proposes methods for producing integrated circuits wherein the dielectric constant between closely spaced and adjacent metal lines is approaching 1. One method of the invention uses low-melting-point dielectric to form a barrier from a void between conductive lines by heating the dielectric. Another method of the invention uses sidewall film to form a similar barrier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for providing animproved dielectric property between adjacent conductive lines in asemiconductor device. More particularly, the present invention relatesto a method and apparatus where the space between closely proximateconductive lines comprises an air void, e.g. a space not filled byeither: (i) the dielectric material usually needed for insulationbetween layers of conductive lines; or 2) the passivation overcoatformed on the top layer of conductive lines. As a result, the inventionprovides for minimum capacitive coupling because the space betweenclosely positioned conductive lines is free space having a unitarydielectric constant.

2. Description of the Related Art

The integrated circuit industry is constantly reducing the separationbetween conductive lines to achieve a smaller integrated circuit.Minimizing integrated circuit area is important because it yields manybenefits including higher reliability, lower cost, and higher speed.

Reducing the spacing between the conductive lines in an integratedcircuit, however, results in an increase in the amount of capacitivecrosstalk between these conductive lines. In conventional integratedcircuits, the interconnect arrangement typically consists of manyadjacent conductive lines. If the capacitive crosstalk between a firstconductive line and a second conductive line is high, then the voltageon the first conductive line alters or affects the voltage on the secondline. This alteration in voltage could cause an integrated circuit tomisinterpret logic levels and/or voltage levels and, therefore,incorrectly process binary and/or analog information. An integratedcircuit that incorrectly processes any information is usually consideredinoperable or faulty.

Illustrated in FIG. 1 is a semiconductor structure having two conductivelines. FIG. 1 depicts a conductive line 1 and a conductive line 2adjacent to conductive line 1. Each of conductive lines 1 and 2 has awidth "w," and conductive line 1 is separated from conductive line 2 bya separation distance "s." A quantity known as a pitch (pitch=w+s) isconventionally used to characterize capacitive crosstalk for adjacentconductive lines used in the integrated circuit industry.

As noted above, a reduction of pitch is an ongoing activity in theintegrated circuit industry to achieve integrated circuits which aremore dense and use substrate surface area more efficiently. Thecapacitance between conductive lines 1 and 2 has been found to increaseexponentially as pitch is reduced or as the conductive lines 1 and 2 arebrought closer together.

This increase in capacitive coupling between conductive lines 1 and 2 isa disadvantage because of the resulting increase in the amount ofcapacitive crosstalk between the conductive lines. For instance, ifconductive line 1 is separated from conductive line 2 by one hundredmicrons, the coupling capacitance will be small and there will bevirtually no capacitive crosstalk between the conductive lines. If thepitch is less than approximately one micron, however, the capacitivecoupling between the conductive lines will be high. Consequently, theamount of capacitive crosstalk will be high and may render theparticular integrated circuit inoperable.

Capacitive crosstalk is a phenomenon wherein a voltage level atconductive line 1 adversely effects and/or alters the voltage level atconductive line 2 through the capacitive coupling between conductivelines 1 and 2. Assume that the pitch in a conductive layer of a DRAM is2 microns and that the DRAM has a plurality of data lines and addresslines. These address and data lines are typically run across theintegrated circuit together or in groups. The conductive lines making upthese data lines and address lines are separated by a minimum pitch toconserve surface area on the integrated circuit. One data or addressline may be carrying a logic one (i.e., a five volt signal). An adjacentdata or address line may be carrying a logic zero (i.e., a zero voltsignal or ground potential).

The capacitive crosstalk due to this small pitch may cause the logic onevalue on the first line to effect and/or alter the logic zero value onthe adjacent line and/or vice versa. Therefore, instead of properlycarrying the correct logic one voltage level or logic zero voltagelevel, the address and data lines may carry an erroneous voltage level.This erroneous voltage level could alter the operation of the DRAMrendering it faulty or inoperable. Crosstalk phenomenon may have an evengreater effect on the operation of analog circuitry.

One prior art method for reducing capacitive coupling, and therefore forreducing capacitive crosstalk between conductive lines having a smallpitch, is the use of low dielectric constant materials to replace theconventional dielectric materials which are typically used forinsulation between conductive layers and the passivation overcoat layerformed on the top conductive layer. Conventional semiconductordielectric materials have a dielectric constant of about four. Some newdielectric materials, such as Teflon may have a dielectric constantbetween about two and four. The use of many dielectric materials havinglow dielectric constants, however, is not feasible because the equipmentnecessary to process properly the new dielectric materials in integratedcircuits is not available. Also, the chemical or physical properties ofmany of these dielectric materials may be difficult to integrate intoconventional integrated circuit processing.

Another prior art method for reducing capacitive crosstalk betweenconductive lines having a small pitch is the use of coaxial cable typearrangements in integrated circuits. A coaxial arrangement typicallyconsists of a first conductor which carries a signal and a secondconductor surrounding the first conductor which is used to shield thefirst conductor from other conductors in the integrated circuit. Thiscoaxial arrangement is very difficult to form in a integrated circuitsince one conductor must be entirely surrounded by a second conductor.Furthermore, this coaxial arrangement requires two or more conductivelayers. Thus, one functional layer of conductive interconnect requiresseveral layers of conductive material. The use of several conductivelayers to form one functional conductive interconnect layer is simplynot a cost effective method for reducing capacitive crosstalk betweenconducive lines with a small pitch.

Another prior art method for reducing capacitive crosstalk betweenconductive lines having a small pitch proposes a partial air gap in thedielectric region between these conductive lines. In this method, a plugis formed between adjacent conductive lines to provide a partial air gapbetween these conductive lines when the interlevel insulation orpassivation overcoat is formed on top of this plug. This method,however, does not provide a material free region between theseconductive lines (i.e. 100% air gap) and will not achieve a dielectricconstant approaching 1. Furthermore, this method entails the use of manyadditional complex and costly processing steps to form the plug and thusis not a simple and cost effective method for reducing capacitivecrosstalk.

For example, if a dielectric layer having a dielectric constant of 4.3is used as an interlevel dielectric, then, if there is no air gap(percentage of air gap=0%), the entire region between the conductivelines depends solely upon the dielectric constant of the dielectricmaterial used; 4.3 in this example. If a 50% air gap is formed betweenconductors, i.e. only 50% of the region between the conductive lines isdielectric material, the effective dielectric constant of the regionbetween the conductive regions is reduced to K =2.75 and, thus, thecapacitance is reduced. The dielectric value K=2.75 is 50% due to airand 50% due to the remaining dielectric regions. Ideally, a materialfree region (100% air gap) is desired wherein the capacitance is verylow and the dielectric constant is reduced significantly.

Yet another prior art method for reducing capacitive crosstalk betweenconductive lines with a small pitch proposes a material free region(100% air gap) between these conductive lines. In this method threelayers including a growth or seed layer in the middle must be formed ontop of the conductive lines. Then the sidewall of the growth or seedlayer is selectively grown to close off the opening between theconductive lines to form the material free region. This method alsoentails the use of many additional, complex, and costly processing stepsand, therefore, is not a simple and cost effective method for reducingcapacitive crosstalk.

Thus, to maintain a small pitch between conductive lines on anintegrated circuit and therefore achieve a denser, lower costingintegrated circuit, a new, simpler and more cost effective method forreducing coupling capacitance, and therefore reducing capacitivecrosstalk between these conductive lines, is needed.

SUMMARY OF THE INVENTION

The invention overcomes the disadvantages and difficulties of the priorart by introducing new methods and devices wherein the area betweenclosely spaced conductors exhibits a dielectric constant approaching 1.To that end, the present invention proposes several methods for leavinga void between closely spaced conductors (usually metal lines) in asemiconductor device. In conjunction with these methods, the inventionalso proposes a semiconductor device wherein there is a void betweenclosely spaced conductors (metal lines).

One method of the current invention calls for creating a barrier aboveclosely spaced conductors (typically metal lines) by using alow-melting-point glass or organic material. More specifically, afterdepositing the conductors, this method calls for depositing a layer oflow-melting-point glass or organic material. Both the glass and theconductor are then etched using a mask and photoresist process.Furthermore, after removal of the photoresist, the wafer is subjected toa heat cycle. The heat cycle is properly adjusted to cause the glass tosag laterally. Where conductors are closely spaced, the sagging glassfrom adjacent lines will much or otherwise form a barrier tosubsequently applied layers. The result is a semiconductor productwherein a void exists between the closely spaced conductors.

Another method of the current invention creates a barrier betweenclosely spaced conductors by using sidewall film. In this method, aconductive layer is deposited and then etched using a mask andphotoresist process. The wafer is then subjected to a heat cycle beforethe photoresist is removed. The heat cycle is properly adjusted to causethe photoresist to sag laterally. Where conductors are closely spaced,the sagging photoresist from adjacent lines with touch or near-touch.The sagging photo resist carries the sidewall film on its formerlyvertical edge. Therefore, being carried by the sagging photoresist, thesidewall film from closely spaced conductors will arc together or neartogether forming a barrier to subsequently applied layers. Thephotoresist is then removed leaving the barrier formed by the sidewallfilm. The result is a semiconductor product wherein a void existsbetween the closely spaced conductors.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 illustrates two conductive lines for the purpose of defining thewidth, separation distance, and pitch of adjacent conductors.

FIG. 2 illustrates a layered wafer section.

FIG. 3 illustrates three conductive lines beating low-melting-pointdielectric and photoresist.

FIG. 4 illustrates three conductive lines beating low-melting-pointdielectric.

FIG. 5 illustrates a finished circuit having a void between closelyspaced conductor lines.

FIG. 6 illustrates a layered wafer section.

FIG. 7 illustrates three conductive lines bearing photoresist andsidewall film.

FIG. 8 illustrates three conductive lines beating laterally expandedphotoresist and sidewall film.

FIG. 9 illustrates a finished circuit having a void between closelyspaced conductor lines.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following describes new methods and devices wherein the area betweenclosely spaced conductors exhibits a dielectric constant approaching 1.This desirable dielectric constant is achieved through various methodsfor leaving a void between closely spaced metal lines in a semiconductordevice. In addition to these various methods, the products that resultfrom these various methods will also be described.

Referring to FIG. 2, area 201 represents all layers of a processedsilicon wafer disposed prior to metal level 202. The term "metal level"as used herein refers to a conductive level of a semiconductor die, asis usually formed of tungsten, aluminum, or other conventionally-knownmaterials. Metal level 202 is disposed on the wafer by any conventionaltechnique, such as, for example, chemical vapor deposition (CVD),sputtering,

After metal level 202 is applied, a low-melting-point dielectric 203 isdisposed on the wafer. The low-melting-point dielectric 203 may be aglass or organic material that will sag before it flows upon exposure toa heat cycle. Examples of satisfactory materials are polystyrene, whichmelts at about 240 degrees Celcius, polypropylene, which melts at about190 degrees Celcius, and various chalcogenide glasses made of S, Se, Te,Sb, Ge, or As, which melt between about 300 and 500 degrees Celciusdepending upon the composition. The manner of disposition of thematerial will typically be selected in response to the material andprocessing considerations. For the above materials, deposition may beperformed by sputtering or spinning as is done for photoresist. Thethickness of the deposition will be functionally related to the linepitch of the specific structure. For structures having a line pitch ofapproximately 1 micron, it is anticipated that dielectric layer 203 willbe formed at a thickness of approximately 2 microns.

Following application of dielectric 203, photoresist 207 is disposed onits surface. Generally, any conventional photoresist will suffice, suchas, for example, OCG 8971 manufactured by Hunt Chemical Group.

After applying the photoresist, the wafer is subjected to masked light206 which is light 205 as filtered through mask 204. This maskingprocess will result in a pattern formation on the surface of photoresist207. The undesirable portions of the photoresist 207, dielectric 203 andmetal 202 may then be etched away. In one preferred embodiment eachlayer will be etched sequentially--photoresist 207, then dielectric 203,then metal 202. However, combinations of metal 202, dielectric 203 andphotoresist may be selected in reference to etchouts to facilitateremoval of two or more of the above layers in a single operation.

Referring now to FIG. 3, there is depicted an exemplary die structure304, depicted in vertical cross section, of a wafer having structuresformed and etched as described above. Structure 304 comprises firstmetal line 303, with low-melting-point dielectric 302 and photoresist301 thereover. Structures 308 and 3 12, including metal lines 307 and311, are constructed in a fashion identical to structure 304, i.e.sections 305 and 309 are photoresist, and sections 306 and 310 arelow-melting-point dielectric. Note also that metal lines 303 and 307 arerelatively closely spaced yet separated by void 313.

In accord with the current method, photoresist sections 301, 305, 309are removed by conventional processes, and the wafer is subjected to aheat cycle. Depending upon the nature of the dielectric sections 302,306, an optimum temperature and time may be determined at whichdielectric section 302 will sag and approach or meet sagging dielectricsection 306 (but neither should flow). The resulting structure willresemble the cross sectional sample shown in FIG. 4. Due to the heatcycle, structures 304 and 308 are now capped by sagging dielectricsections 302 and 306 respectively. Furthermore, the sagging dielectricsections 302 and 306 have substantially encapsulated void 313. Moreover,even if there is not a perfect seal between sagging dielectric 302 and306, a seal will be made when the metal level wafer is passivated (orinsulated) with additional dielectric material. Some attention should begiven to use a passivation material that will serve to seal any openingswithout flowing through into the void area. The final cross section isshown in FIG. 5 after passivation 501 has been applied.

In an alternative method of practicing the present invention, a sidewallfilm may be used to form a barrier that prevents dielectric fromentering the void between closely spaced metal lines. Referring now toFIG. 6, area 601 schematically represents all layers of a processedsilicon wafer disposed prior to metal level 602. Metal level 602 isagain disposed on the wafer by any conventional technique. After metal602 is deposed, an appropriate photoresist layer 603 is disposed on itssurface.

After applying the photoresist, the wafer is subjected to masked light606 which is light 605 as filtered through a mask 604. This maskingprocess will again result in a pattern formation on the surface ofphotoresist 603. The undesirable portions of the photoresist 603 andmetal 602 may then again be etched away.

One envisioned embodiment provides for both chemical and physicaletchings of the wafer structure. Generally, most conventional chemicaletching processes will result in a polymer film covering the wafersurface. The polymer film is generally the result of the unavoidable mixof etching chemicals with etched metal. The etching of polysilicon usingechtants containing chlorine is known to produce a film of this typewhen the photoresist is present. Accordingly, absent a physical etch,the polymer film would cover both horizontally and vertically disposedsurfaces of the wafer. The physical etch, however, generally comprisesvertically directed ion bombardment. This vertical ion bombardmentremoves the polymer film from all horizontally disposed surfaces.Therefore, the only remaining polymer film exists on verticalsurfaces--thus the name "sidewall film."

Referring now to FIG. 7, therein is depicted an exemplary die structure701, illustrated in vertical cross section, for a wafer formed andetched as described relative to FIG. 6. Structure 701 comprises metalline 706, photoresist 704 and sidewall films 713 and 710. Structures 702and 716 are constructed in a fashion identical to structure 701, i.e.sections 708 and 705 are photoresist, sections 711, 712, 714 and 715 aresidewall films, and sections 707 and 709 are conductive (typicallymetal) lines. Again, metal lines 706 and 707 are relatively closelyspaced yet separated by void 703.

According to one method, the wafer is subjected to a heat cycle prior toremoval of photoresist sections 704, 705, 708. The photoresist sections704, 705, 708, will be chosen such that a ratio of temperature and timemay be determined by which photoresist 704 will sag and meet (or nearlymeet) sagging photo resist 705, without flowing. Accordingly, theprocess is controllable to yield die structures as depicted in the crosssection sample in FIG. 8.

Referring now to FIG. 8, due to the heat cycle, structures 704 and 705now comprise sagging photoresist sections 704 and 705 respectively.Furthermore, the sagging photoresist sections 704 and 705 have alteredthe shape of sidewall films 710 and 712 causing those films tosubstantially meet and encapsulate void 703. Thereafter, photoresistsections 704 and 705 are removed leaving sidewall films 710 and 712 toencapsulate the void 701. This etch is preferably performed with gentledry process such as that available in a "down stream" dry resist removaloperation. Of course, virtually any etching is appropriate which doesnot remove the polymer sidewall film 710 and 712.

Following removal of the photoresist 704, 705, the upper surface of thewafer is passivated or insulated with a dielectric material 901 as shownin FIG. 9. Therefore, even if there is not a perfect seal betweensidewall films 710 and 712, a seal will be made when the metal levelwafer is passivated (or insulated) with additional dielectric material901. Some attention should be given to use a passivation material thatwill serve to seal any openings without flowing through into the voidarea 703.

What is claimed is:
 1. A method of building a semiconductor device,comprising the steps of:forming a layer of conductive material on awafer; forming a layer of dielectric material on top of said conductivelayer; etching said layer of dielectric material and said layer ofconductive material to form a first line and a second line adjacent tosaid first line, each said line havinga conductive section, and adielectric section disposed upon said conductive section; and heatingsaid wafer until said dielectric sections move toward each other forminga cavity between the conductive section of said first line and theconductive section of said second line.
 2. The method of claim 1 furthercomprising the step of disposing a layer of insulating material uponsaid wafer after the formation of said cavity.
 3. The method of claim 1wherein said dielectric material is glass.
 4. The method of claim 1wherein said dielectric material is an organic material.
 5. The methodof claim 1 wherein said conductive material is metal.
 6. The method ofclaim 1, wherein said dielectric material and said layer of conductivematerial are etched in a single etching operation.
 7. A method ofbuilding a semiconductor device, comprising the steps of:forming a layerof conductive material on a wafer; forming a layer of dielectricmaterial on top of said conductive layer; etching said layer ofdielectric material and said layer of conductive material to form afirst line and a second line in spaced relation to said first line, saidfirst line havinga conductive section, and a dielectric section disposedupon said conductive section; and heating said wafer until saiddielectric section moves toward said second line defining a cavitybetween the conductive section of said first line and the conductivesection of said second line.
 8. The method of claim 7 wherein saidconductive material comprises a metal.
 9. A method of building asemiconductor device comprising:forming a layer of conductive materialon a wafer; forming a layer of photoresist upon said conductive layer;etching said layers of conductive and photoresist materials to form afirst line assembly and a second line assembly, each of said first andsecond line assemblies having a conductive section and a photoresistsection disposed thereon, each of said first and second line assembliesalso having at least one sidewall, said sidewall carrying a sidewallfilm; and heating said wafer until said photoresist sections movelaterally causing two adjacently oriented sidewall films to move towardeach other and form a cavity between said conductive sections.
 10. Themethod of claim 9 further comprising the step of removing saidphotoresist layer without removing said sidewall film.
 11. The method ofclaim 10 wherein a down stream dry resist removal tool is used to removesaid photoresist layer.
 12. A method of forming a semiconductor device,comprising the steps of:providing a substrate; forming a layer ofconductive material on said substrate; forming a layer of photoresistmaterial over said conductive layer; forming a pattern of conductivematerial and photoresist material on said substrate, said patterncomprising a plurality of spaced conductive lines with photoresist oversaid lines, said patterned materials having side surfaces, definingvoids between said patterned materials; forming a film on said sidesurfaces; heating said wafer until said photoresist changes conformityand moves a portion of said film on said side surfaces to at leastpartially enclose said voids between said spaced conductive lines. 13.The method of claim 12 further comprising the step of removing saidphotoresist layer without substantially removing said sidewall film.